Monday 23 March 2009
A number of minerals are essential for health. Calcium and phosphorus are required in large amounts and are considered in the discussion of vitamin D. Trace elements are metals that occur at concentrations smaller than 1 μg per gram of wet tissue. Of the various trace elements found in the body, only five-iron, zinc, copper, selenium, and iodine-have been associated with well-characterized deficiency states.
In theory, a deficiency of a trace element might occur for many of the same reasons as a vitamin deficiency does, but three influences are particularly relevant:
(1) inadequate supplementation in preparations used for total parenteral nutrition;
(2) interference with absorption by dietary constituents;
(3) inborn errors of metabolism leading to abnormalities of trace metal absorption.
Dietary interference as a mechanism was first noted among inhabitants of Egypt and Iran who subsisted largely on unrefined cereals; sufficient phytic acid and fiber were present in the diet to bind zinc and block its absorption. Genetic malabsorption syndromes involving a trace element are rare. In one, failure to synthesize metallothionein (a metal-binding protein) in intestinal mucosal cells blocks absorption of both copper and zinc.